Various bacteria and fungi are known to survive on and degrade wood. For example, white-rot basidiomyctes (fungi) are known to form and survive on wood, and their nourishment as obtained from the wood is manifested in the observed slow decay of the wood. Such fungi typically obtain their nourishment from the ultimate degradation products of cellulose but release a host of enzymes which degrade substantially all organic wood components including not only the cellulose but also hemicellulose and lignin.
White-rot basidiomycetes remove lignin from wood in several morphologically distinct patterns. One type of decay known as "selective delignification" is apparent when greater amounts of lignin are degraded relative to the amount of cellulose. In this type of decay, lignin in the secondary wall and middle lamella may be almost entirely removed, whereas large quantities of cellulose in the S2 layer of the cell wall are left intact. White rot basidiomycetes can also cause a "simultaneous rot". This type of decay is characterized by the removal of both cellulose and lignin, leaving cells either fiddled with bore holes and erosion troughs, or with extensively thinned secondary walls. Much variation exists among these decay types. Some Basidiomycetes cause only a simultaneous rot whereas others may produce a simultaneous rot in one part of the substrate and predominantly a delignification in another. In such cases, a chemical analysis of the entire substrate can misrepresent the potential of these fungi to cause selective lignin removal. Some other white rot fungi have been shown to be initially very selective for lignin and then later attack the remaining cellulose. Thus, the selectivity of some fungi for lignin can change depending on the stage of decay at which chemical analyses are done.
To avoid difficulties arising from the interpretation of lignin and wood sugar analyses, several methods are used to evaluate the capacity of fungi to selectively degrade lignin. A screening technique involving scanning electron microscopy has been used to differentiate the type of decay produced by different white rot fungi. This technique, coupled with traditional chemical analyses, provides the more accurate means of screening available.
One such type of white rot fungi, Phanerochaete chrysosporium and mutants thereof (see, for example, U.S. Pat. No. 4,687,741) can be viewed with simplicity as releasing a battery of largely cellulose degrading enzymes (cellulases) in order to obtain its nourishment from wood. Since the cellulose is isolated within barrier layers or regions made up of hemicellulose and lignin, the supply of cellulose directly available to the fungus is limited and becomes exhausted. At this point the fungus has less and less cellulose available for survival and enters essentially a starvation phase in which the fungus then releases hemicellulose and lignin degrading enzymes (hemicellulases and ligninases, respectively) which remove the barrier layers and expose the next layered supply of cellulose for nourishment. This cycle will be repeated on and on in the process of the total natural decay of wood. Essentially on the theory that the fungus could be induced to produce enzymes affecting lignin in deference to those affecting cellulose, Phanerochaete chrysosporium cultures were found to degrade lignins in pulps, including chemically modified pulps (known as chemical pulps), in a selective manner. The fungus was also found to degrade chemically altered lignin degradation products as produced, for example, in the primary bleaching stage of the kraft process (so-called "E-1 effluents"). P. chrysosporium is more effective on hardwood lignins than on softwoods.
One primary chemical method for making pulp from wood involves the digestion of lignin in the wood with sodium sulfide and sodium hydroxide. This is termed the sulfate or kraft process, and the resulting pulp is termed "chemical pulp". The known "sulfite" process also produces a chemical pulp.
Wood pulp produced, for example, in the kraft process, generally contains about 4-12% by weight of residual lignin, at least some of which is chemically modified to give the pulp a characteristic brown color. To obtain a pulp of high brightness and brightness stability, the lignin and/or modified lignin must be further treated and removed by certain agents in one or more treatments or stages commonly referred to as bleaching. And, in general, all chemical pulps are subjected to one or more bleaching treatments for similar purposes. Many bleaching processes exist but almost all kraft pulp bleachings begin with the chlorination-extraction (C-E) stage. Other bleachings use hypochlorite, chlorine dioxide, oxygen or other oxidizing chemicals such that the degraded lignin component freed from the pulp is chlorinated and/or oxidized, and hence further chemically modified. There is some loss and damage of cellulosic fibers during the C-E stage but the cellulosic content remains high and sufficiently intact that paper/board products of acceptable quality can be produced, hence the process is selective. The C-E effluents (included among those called E-1 effluents) resulting from treated pulp contain a very large number of organic compounds having a bound chlorine content of 2.5-3.5 kg/ton pulp. Some of these compounds, primarily the chlorinated phenolics, have been identified as having toxic, mutagenic and carcinogenic effects (Alberti, B. N. and Klibanov, A. M. [1981] Biotechnology and Bioengineering Symp. 11:373-379). Alternatives to chlorine bleaching have, therefore, long been sought by industry. As use herein, the term E-1 effluent refers to the effluent resulting from extraction of chemical pulp after the first or primary bleaching stage.
The use of fungi and their enzyme systems in the processing of wood pulps and chips (biopulping generally) to improve efficiency and reduce environmental problems in the pulp and paper industry has been the subject of increasing interest and investigation.